In the field with which the invention is concerned, the transmission of messages is one-way and, from the receiver side, it is impossible to affect transmission from the transmitter and to request and obtain the repetition of messages. The present state of the art with respect to radio reception and receivers, particularly with regard to receivers for personnel location, is described inter alia in Patent Application GB No. 2,101,779A, published in January 1983, as a personnel locator adapted for receiving calls sent by radio and for receiving and storing a plurality of messages and for presenting one message at a time in a display window. The received message is compared with each of the already stored messages and is stored in the receiver's memory only if it differs from all the messages already stored. A further case where repeated messages may be received is when the receiver is in a boundary area where the reception area associated with two time-displaced radio transmitters situated adjacent each other overlap each other; the same message can then be received first from one transmitter and then from the second transmitter.
With the intention that the probability of a message's being correctly received should be increased, the message is sent repeatedly, according to the above-mentioned British Patent Application, but the mentioned comparison prevents its being stored more than once. A weakness in the thus described personnel locator is that if a character in one of the received messages has been received in a distorted condition, the comparison cannot show that the same message has been received, and different versions of the same message will be stored twice and possibly several times.
There is further described an example of a method for transmitting and receiving personnel location calls in "Final Report of the British Post Office Code Standardisation Advisory Group (POCSAG)", London, 1978.